In spite of making great strides in computers, in particular in personal computers from the middle of the 1990s, there have been limited changes in their peripheral equipment. However, the peripheral equipment of personal computers or workstations is noticeably changing. Some such changes are due to new general-purpose buses, for example, USB (universal serial bus), FW (fire wire, or IEEE1394), FC (fiber channel), SSA (serial storage architecture), and so on. The USB is expected to be the next generation computer peripheral equipment interface, with the FW (or, IEEE1394) being appropriate for multi media use.
Unlike the conventional parallel buses, the USB has the following characteristics. It does not need to be set up by a terminator or jumper in the circumstance of PnP (plug-and-play). Also, auto assignment of ID and a hot plug, i.e., a device is detachable when the computer is in a power-on state, are possible. Moreover, the USB cable has only four lines, i.e., two signal lines D+(GREEN), and D−(WHITE), power supply line VBUS (RED), and ground line GND (BLACK). Thus, it is possible to fabricate short cables and small connectors, resulting in decreasing production cost as well as developing inexpensive peripheral equipment.
According to the “USB Specification Revision 2.0” (Apr. 27, 2000), the USB cable connects USB devices to a USB host. There is only one host in any USB system. The USB system has a tiered star topology. The USB devices are hubs providing additional connections for the USB system and functions providing capabilities for the USB host such as ISDN (integrated service digital network) connection, digital microphone, keyboard, digital joystick, speaker, etc. The host is a host computer system where a host controller is installed for achieving the USB interfacing operation of the host, and necessarily has a root hub being directly connected to the host controller. A plurality of nodes, i.e., other hubs or function devices are connected to one hub. Data being transferred between functions passes through the host.
According to the USB Specification Revision 2.0 (Apr. 27, 2000), the USB operation in a high speed mode supports data transmission of 480 Mb/s. Further, a low speed mode and full speed mode support the data transmissions of 1.25 Mb/s and 12 Mb/s, respectively.
A transmission envelope detector is referred to as “squelch” operates in the high speed mode. Generally, the squelch detection circuit serves to detect low differential input voltage level and detects whether the data being transmitted on the bus is a noise element or a valid signal element.
According to the USB Specification Revision 2.0 (Apr. 27, 2000), the differential voltage formed between the signal lines D+(GREEN) and D−(WHITE) is used for three purposes. First, when differential receiver on a receiving end of the cable receives a differential data signal, the differential receiver utilizes a squelch detector to detect whether the signal of the connector is invalid. Secondly, a differential envelope detector on the receiving end of the cable measures when the link is in a squelch state. Thirdly, in a case of a downstream transceiver, the differential envelope detector monitors whether the signal of the connector on the connector is in a high speed state.
In accordance with the USB Specification Revision 2.0 (Apr. 27, 2000), the transmission envelope detector serves to represent that the data is invalid when a voltage level of the differential signal on the input ends of the receiver is lower than a high speed squelch level, referred to as a “squelch threshold”. It is desirable that the transmission envelope detector represent the squelch when the differential signal voltage level is less than 100 mV and represent that the line is not in the squelch state when the differential signal voltage level is more than 150 mV.
In general, the conventional squelch detection circuit detecting the cases that voltage level of the differential input signal is less than 100 mV or more than 150 mV includes a comparator. The comparator provides a low level output when the signal is less than 100 mV, and a high level output when the signal is more than 150 mV. However, the conventional squelch detection circuit is subject to be in a high-impedance state at a cross point of two differential input signals. Here, the cross point is a point where two time variant differential input signals meet each other. As a result, the conventional squelch detection circuit employing the foregoing comparator cannot detect whether the transmitting data is a noise element or a signal element at the cross point of the two differential input signals.